As a subclass of commercially available polymers, polyurethane elastomers have several properties whose advantages confer unique benefits on these products. Typically, polyurethanes show high abrasion resistance with high load bearing, excellent cut and tear and resistance, high hardness, resistance to ozone degradation, yet are pourable and castable. Compared to metals, polyurethanes are lighter in weight, less noisy in use, and show better wear and excellent corrosion resistance while being capable of cheap fabrication. Compared to other plastics, polyurethanes are non-brittle, much more resistant to abrasion, and exhibit good elastomeric memory. Polyurethanes find use in such diverse products as aircraft hitches, bushings, cams, gaskets, gravure rolls, star wheels, washers, scraper blades, impellers, gears, and drive wheels.
Part of the utility of polyurethanes derives from the enormous diversity of properties in products which can result from a relatively limited number of reactants. Typically, polyurethanes are prepared on site by curing a urethane prepolymer, which are adducts of polyisocyanates and polyhydric alcohols. A large class of such prepolymers are approximately 2:1 adducts of a diisocyanate, OCN--Y--NCO, and a diol, HO--Z--OH, whose resulting structure is OCN--Y--NH--CO.sub.2 --Z--O.sub.2 CNH--Y--NCO. Although Y is susceptible of great variety, usually being a divalent alkyl, cyclohexyl, or aromatic radical, in fact the most available prepolymers are made from toluene-2,4-diisocyanate (TDI) or methylene-4,4'-diphenylisocyanate (MDI). The diols used display a greater range of variety; Z may be a divalent alkyl radical (i.e., an alkylene group), and the diols frequently are ethers or esters which are the condensation products of glycols with alkylene oxides or dicarboxylic acids, resp. The polyurethane elatomers are formed by curing the prepolymer.
Curing involves the reaction of the terminal isocyanate groups of the prepolymer with active hydrogens of a polyfunctional compound so as to form high polymers through chain extension and, in some cases, crosslinking. Diols, especially alkylenediols, are the most common curing agents for MDI-based prepolymers, and where such diols have the structure HO--X--OH, where X is an organic moiety, most usually an alkylene group, the resulting polymer has as its repeating unit, EQU (--Y--NHCO.sub.2 --ZO.sub.2 CNH--Y--NHCO.sub.2 --X--O--CONH--)
Where a triol or a higher polyhydric alcohol is used crosslinking occurs to afford a nonlinear polymer.
Although other polyfunctional chemicals, especially diamines, are theoretically suitable, but with a few exceptions none have achieved commercial importance as a curing agent. The major exception is 4,4'-methylene di-orthochloroaniline, usually referred to as MOCA, a curing agent which is both a chain extender and a cross linker. TDI-based prepolymers typically are cured with MOCA, and the resulting products account for perhaps most of the polyurethane elastomer market. One reason that polyhydric alcohols generally have gained acceptance as curing agents is that their reaction with urethane prepolymers is sufficiently fast to be convenient, but not so fast as to make it difficult to work with the resulting polymer. In casting polymers it is desirable that the set-up time be reasonably short, yet long enough for the material to be cast into molds. This property is conventionally referred to as pot life. Generally speaking, diamines react with prepolymers, and especially MDI-based prepolymers, so quickly that they are not usable as curing agents. However, primary aromatic diamines with electronegative groups on the aromatic ring, or with alkyl groups ortho to the amino moiety, exhibit sufficiently decreased reactivities with some prepolymers as to afford a desirable pot life, hence the use of, for example, MOCA as a curing agent for TDI-based prepolymers. However, MOCA and other of the aforementioned diamines still remain too reactive to be used, for example, with MDI-based prepolymers.
Previously only primary aromatic diamines seem to have been investigated as curing agents. Presumably this is because secondary diamines were expected to have an unacceptably long pot life, and because they could act only as chain extenders in contrast to the crosslinking capabilities of primary diamines. It was then quite surprising and unexpected to find that the secondary aromatic diamines of this invention gave an acceptable pot life with a variety of urethane prepolymers. By way of definition, for the purpose of this application, a chain extender is a curing agent which increases the length of individual chains, and a cross linker is a curing agent which links two individual chains. Thermoplastic properties usually are associated with chain extension, and rigidity and compression set are properties associated with crosslinking.
U.S. Pat. No. 3,846,351 describes the quite narrow use of secondary aromatic alkyl diamines in combination with polyols as catalysts and chain extenders in the production of flexible polyurethane foams. It is important to recognize that such secondary diamines were used only in combination with polyols, preferably at the relatively low level of 0.5-5 parts per 100 parts of polyol. In an examination of aromatic diamines as chain extenders in polyurethane elastomers by reaction injection molding, the sole secondary aromatic diamine examined, N,N'-diisopropyl-4,4'-methylenedianiline, afforded products whose properties were generally unacceptably inferior to those exhibited by polyurethanes made with primary aromatic diamines as curing agents. "D. Nissen and R. A. Markovs," Proceedings of the 27th SPI Annual Technical/Marketing conference, 71-8. Consequently, it seems fair to say that the prior use of secondary aromatic diamines as a curing agent in polyurethane manufacture is quite limited in scope and generally unfavorable in results.
Therefore, it was particularly surprising to find that certain N,N'-dialkyl-4,4'-methylenedianilines are generally effective curing agents for a broad range of urethane prepolymers. The resulting polyurethanes often have the advantage of being thermoplastic rather than thermosetting, thereby making them especially useful as coatings, adhesives, and sealants. The secondary aromatic diamines of this invention have commercially acceptable pot lives as curing agents for many prepolymers, and afford products with an impressive variety of properties.